Double-clutching is usually reserved for downshifting...
As far as upshifting goes, you have to wait a little longer between gears for the input shaft to slow down so that it more nearly matches the speed of the next-higher gear you're going to...
Sounds like you need a clutch/throw-out bearing and/or linkage repair/adjustment...
If that clutch doesn't completely disengage, upshifting is going to be REALLY difficult.
I don't know if any of the '47 Art Deco trucks had the SM-420 synchro 4-speed...I think they didn't show up until the Advance Design trucks were introduced...
The crash boxes are realy easy to ID: the have the little "pinky-lever" for the Reverse lock-out.
My '41 dump has the old crash-box...what fun !
My former '48 IHC KB-2 pick-up also had a four-speed crash-box; boy what noises that made in second and third: somewhere between a moan and a wail !
Top gear is quiet only because you're not running through any gear reduction !
Fix your clutch first, then you'll have a better time learning to drive crash-boxes !
Good luck !
